Numerical study on the flow control mechanisms of an off-surface circular cylinder for wall-bounded flows with separation

This paper focuses on the off-surface flow control with a circular cylinder. Numerical investigation of the effectiveness and mechanisms of this method for flow separation control is conducted using two benchmark test cases. They are turbulent flow over a backward-facing step and the NASA wall-mounted hump separated flow. Firstly, Reynolds-averaged Navier-Stokes simulations are performed to analyze the influence of position and diameter of the cylinder on flow reattachment. The optimal control parameters for the cylinder are determined by a parametric study. Then, delayed detached eddy simulations are carried out for the separated flows under control using a cylinder with the optimal parameters, in order to investigate the flow control mechanisms in depth. Numerical simulation results show that an off-surface circular cylinder can significantly affect turbulent flow separation and reattachment. When the cylinder is placed at a suitable location outside the core region of the separated shear layer, the shear layer instability characteristics are enhanced with reduced sizes of turbulent vortex in the both reattachment zone and the recirculation region. With the application of control, the curving degree of the separated shear layer downward the lower wall is slightly increased, and the flow reattachment point moves upstream with a reduced recirculation region.